In general, in the production processes of high-density semiconductor integrated circuits such as LSI, color filters for CCD (charge-coupled device) and LCD (liquid crystal display device), magnetic heads, etc., microfabrication utilizing the photolithographic technique using photomasks is performed.
In this microfabrication, a photomask, in which a light-shielding film consisting of a metal thin film such as a chromium film is generally formed on a light transmissive substrate made of quartz glass, aluminosilicate glass or the like by means of sputtering, vacuum deposition or the like to provide a photomask blank, wherein the light-shielding film is formed to have a predetermined pattern, is used.
A photomask is produced using such a photomask blank with the following processes: an exposure process in which a desired pattern exposure is applied to a resist film formed on the photomask blank; a development process in which, after the desired pattern exposure is applied to the resist film formed on the photomask blank, a developing solution is supplied thereto to dissolve portions of the resist film soluble in the developing solution, thereby forming a resist pattern; a etching process in which, using the obtained resist pattern as a mask, portions in which a light-shielding film is exposed with the resist pattern not formed are removed by etching, such as, wet etching using an etching solution consisting of a mixed aqueous solution of ceric ammonium nitrate and perchloric acid, and dry etching using chlorine gas, thereby forming a predetermined mask pattern on a light transmissive substrate; and a stripping/removing process in which the remaining resist pattern is stripped and removed.
During patterning of the light-shielding film in the etching process, the resist pattern formed on the light-shielding film must remain with a sufficient film thickness. However, when the resist film thickness is increased, the aspect ratio is increased, and this causes the problem of pattern collapsing, etc., particularly in the case where a fine pattern is to be formed. Therefore, in order to miniaturize a mask pattern formed on a photomask, it is required to decrease the thickness of a resist film formed on a photomask blank.
Regarding this point, Japanese Laid-Open Patent Publication No. 2007-33470 (Patent Document 1) discloses a photomask blank comprising a light-shielding film having a thickness of 100 nm or less, wherein the film has a structure in which the percentage of the film thickness of a chromium-based compound having a high etching rate is 70% or more to enable reduction in etching time, thereby realizing miniaturization of the resist. Specifically, Patent Document 1 discloses a photomask blank in which a semitransparent film, a CrON film, a Cr film and a CrON film are laminated on a light transmissive substrate, wherein the percentage of the thickness of the CrON film is 70% or more.
However, regarding the above-described CrON film, it is just that the optical density per unit film thickness at a wavelength of 450 nm is set, and regarding a wavelength of exposure light equal to or less than wave length of an ArF excimer laser light, no optimization has been made. In particular, in the case of hyper-NA lithography, the angle of light incidence relative to a photomask becomes shallower, and this causes the problem that a miniaturized mask pattern itself shades a transfer image (shadowing). When a light-shielding film is thick, reduction of the amount of light (deterioration of contrast) due to shadowing is highly influential. In addition, the cross-section shape is prone to vary, and this, together with shadowing, causes reduction of transfer accuracy of CD (Critical Dimension).
Further, in order to improve resolution performance by finer processing of a photomask, a phase shift mask blank, in which, for example, a phase shift film having the transmittance of several percent to several tens percent with the phase of transmitted light being shifted 180° (e.g., a phase shift film made of a metal silicide oxide film or a metal silicide oxide nitride film described in the specification of Japanese Patent No. 2837803 (Patent Document 2), a metal silicide nitride film described in the specification of Japanese Patent No. 2966369 (Patent Document 3) or the like) and a light-shielding film such as a chromium film having etching selectivity relative to the phase shift film are formed on the above-described light transmissive substrate by means of sputtering, vacuum deposition or the like, and a phase shift mask in which these light-shielding film and phase shift film are formed in a predetermined pattern are used.
Regarding the structure of the phase shift mask, a general example in the case where the transmittance of the phase shift film is 10% or more (e.g., 10% to 40%) is described in the specification of Japanese Patent No. 3445329 (Patent Document 4; Example 1 and FIG. 1), and this is a phase shift mask having a structure in which a light-shielding film pattern is formed on a phase shift film pattern formed within a pattern transfer region and a light-shielding film with a width of a predetermined value or more is formed in a non-pattern-transfer region. Further, a general example in the case where the transmittance of the phase shift film is less than 10% (e.g., 2 to less than 10%) is described in the specification of Japanese Patent No. 3411613 (Patent Document 5; Example 1 and FIG. 1), and this is a phase shift mask having a structure in which no light-shielding film pattern is formed on a phase shift film pattern formed within a pattern transfer region and a light-shielding film with a width of a predetermined value or more is formed in a non-pattern-transfer region.
As recited in claims 25 to 29 in International Publication WO 2004/090635 pamphlet (Patent Document 6), a phase shift mask blank may have a structure in which a film for etching mask made of an inorganic material having resistance to dry etching of a light-shielding film is laminated on the light-shielding film, which is formed on a phase shift film and comprises chromium.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2007-33470    [Patent Document 2] Japanese Patent No. 2837803    [Patent Document 3] Japanese Patent No. 2966369    [Patent Document 4] Japanese Patent No. 3445329    [Patent Document 5] Japanese Patent No. 3411613    [Patent Document 6] International Publication WO 2004/090635 pamphlet